The invention relates to a DRAM (Dynamic Random Access Memory) cell including a storage capacitor device and a selection transistor, which are provided at least partly in a semiconductor body.
As is known, DRAM memory cells can store the information contained in them only for a specific time. This is because the information is present in the memory cell as an electric charge in a storage capacitor having a specific capacitance.
The loss of charge and hence information in the storage capacitor takes place through various mechanisms, such as leakage currents through the dielectric of the storage capacitor, stray currents between the capacitor electrodes and other conductors which are at a potential that differs from the potential of the capacitor electrodes, etc.
If the capacitance of a storage capacitor could theoretically be increased as desired, then the charge in this storage capacitor could also be preserved for as long as desired.
Conversely, the time period during which the charge is retained in a storage capacitor decreases with a decreasing capacitance of the capacitor. Since the capacitance of a capacitor is essentially proportional to its dimensions, its size significantly influences the information storage time.
However, the size of storage capacitors is continually being reduced with ever smaller structural dimensions. Through skillful dimensioning of trench cells accommodating storage capacitors, it has been possible hitherto, despite the diminishing structural dimensions, to keep the storage time essentially constant, that is to say to compensate for the reduced structural dimensions. Solutions for this goal are achieved with deeper trenches, thinner dielectrics of the storage capacitors, dielectrics having higher dielectric constants, etc.
Nevertheless, limits are also imposed on the depth of the trenches in conjunction with a smaller diameter or the widening of the trenches deep in the semiconductor material (xe2x80x9cbottled trenchesxe2x80x9d) or the introduction of dielectrics having higher dielectric constants. Moreover, the complexity of the fabrication process increases if, for example, special dielectrics having higher dielectric constants or trenches widened in their depth are used in order to compensate for the reduction in capacitance accompanying the miniaturization of the structural dimensions.
It is accordingly an object of the invention to provide a DRAM memory cell which overcomes the above-mentioned disadvantages of the heretofore-known memory cells of this general type and with which, even in the case of advancing miniaturization of the structural dimensions, the reduction in capacitance can be counteracted, thereby making it possible to prevent a decrease in the storage time.
With the foregoing and other objects in view there is provided, in accordance with the invention, a DRAM memory cell, including:
a semiconductor body;
insulating layers disposed on the semiconductor body, the insulating layers having contact holes formed therein;
a storage capacitor device provided at least partly in the semiconductor body, the storage capacitor device including at least a first storage capacitor and a second storage capacitor connected in parallel with one another;
the first storage capacitor being a trench capacitor disposed in the semiconductor body;
the second storage capacitor being a stacked capacitor disposed on the semiconductor body such that the second storage capacitor is located above the first storage capacitor;
the second storage capacitor being electrically connected to if the first storage capacitor via the contact holes in the insulating layers; and
a selection transistor provided at least partly in the semiconductor body.
In other words, the object of the invention is achieved by virtue of the fact that the storage capacitor device includes at least two storage capacitors connected in parallel with one another, of which one is provided in the semiconductor body and the other is provided on the semiconductor body.
In the case of the DRAM memory cell according to the invention, two storage capacitors, one of which is preferably a trench storage capacitor, while the other storage capacitor is formed by a stacked capacitor on the semiconductor body, are connected in parallel with one another, so that the capacitance of the stacked capacitor formed on the surface of the semiconductor body contributes to an increased capacitance of the storage capacitor device. The trench storage capacitor can be introduced into the semiconductor body in a customary manner in accordance with the trench concept through the use of an etching step, while the stacked capacitor on the surface of the semiconductor body includes at least two capacitor electrodes which lie parallel to one another and are isolated from one another by a dielectric. In order to enlarge the electrode area, the electrodes may also be xe2x80x9cintermeshedxe2x80x9d in one another and in this case have a tree-like structure, for example. The stacked capacitor is preferably provided above the trench capacitor, so that no additional area is taken up for the stacked capacitor.
The two capacitors are connected, in parallel with one another, to the selection transistor, whose gate is connected to a word line and whose drain has a bit line contact to a bit line. In this case, the selection transistor may be embodied either laterally or vertically. The source of the selection transistor is connected to a node between the two storage capacitors connected in parallel with one another. The other two electrodes of the two storage capacitors may be connected to ground.
The storage capacitor device preferably includes two storage capacitors connected in parallel with one another. However, it is also possible, of course, for three or more, rather than two, storage capacitors also to be connected in parallel and jointly connected to a selection transistor in order thus to further increase the capacitance of the storage capacitor device of the DRAM memory cell.
The semiconductor body is preferably composed of silicon. However, it is also possible, of course, to use other semiconductor materials, such as, for example, AIIIBV semiconductor materials or SiC.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a DRAM memory cell, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.